MINUTES  OF  PROCEEDINGS  OF  THE  INSTITU- 
TION OF  CIVIL  ENGINEERS,  LONDON, 
ENGLAND,  SESSION  OF  1862-63. 


AMERICAN  IRON  BRIDGES. 


ABSTRACT  OF  THE  DISCUSSION 


UPON*  A PAPER  SUBMITTED 

By  ZERAH  COLBURN. 


EDITED  BY 

CHARLES  MANBY,  F.  R.  S.,  M.  Inst.  C.  E„ 

HONORARY  SECRETARY. 

AND 

JAMES  FORREST,  Assoc.  Inst.  C.  E., 

SECRETARY. 


NEW  YORK: 

D.  VAN  NOSTRAND,  192  BROADWAY, 


1 8 6 7. 


^ Digitized  by  the  Internet  Archive 
in  2017  with  funding  from 

University  of  Illinois  Urbana-Champaign  Alternates 


https://archive.org/details/americanironbridOOcolb 


INSTITUTION  OF  CIVIL  ENGINEEES. 


May  .5,  1863. 

JOHN  HAWKSHAW,  President, 
in  the  Chair. 

Mr.  Zerah  Colburn  submitted  a Paper  describing  the 
several  plans  of  Iron  Bridges  in  use  upon  the  Bailways  of 
the  United  States,  illustrated  by  a series  of  diagrams. 

No.  1,091. — “ American  Iron  Bridges.”  By  Zerah  Colburn. 

After  reading  the  Paper,  Mr.  Colburn  said,  he  had  re- 
frained from  entering  more  into  detail  to  avoid  making  it 
tedious,  but  he  should  be  happy  to  reply  to  any  questions. 
No  doubt,  in  looking  at  the  diagrams,  it  would  be  considered 
that  American  engineers  had  practised  great  economy  in  de- 
signing these  bridges.  The  fact  was,  that  if  bridges  could 
not  be  constructed  of  iron  at  a moderate  cost,  the  Bailway 
Companies  in  the  States  would  not  adopt  them,  but  would 
continue  to  use  timber  bridges,  which  could  be  built  at  from 
£5  to  £7  per  lineal  foot.  It  was  on  the  score  of  cost,  alone, 
he  believed,  that  American  engineers  had  adopted  cast  iron 
for  all  part^  in  compression  ; so  many  square  inches  of  sec- 
tion could  be  put  into  the  tubular  form  for  one-third  the  cost 
in  cast-iron  that  would  be  incurred  with  wrought-iron  plates. 
In  the  case  of  the  Green  Biver  Bridge,  the  top  chords  and 
the  vertical  posts  were  of  cast-iron  pipes.  Care  was  requi- 
site in  casting  the  pipes.  Mr.  Fink  tested  the  iron  for  all  the 
pipes,  and  holes  were  drilled  in  the  side  that  lay  uppermost 
in  the  sand,  to  ascertain  that  the  cores  had  not  floated,  and 
thus  that  the  metal  was  of  uniform  thickness.  The  fact  of 
cast-iron  being  used  in  the  top  chords  was  a sufficient  expla- 
nation why  bridges  were  not  made  continuous  over  two,  or 
more,  spans  in  America.  In  nearly  all  these  bridges  almost 
the  whole  of  the  iron  was  made  to  do  work  in  carrying  the 
load.  They,  no  doubt,  differed  from  what  English  engineers 
were  accustomed  to,  and  he  feared  the  diagrams  would  pre- 
sent an  extraordinary  appearance  to  English  eyes  ; but  they 
were  faithful  representations. 

Mr.  Colburn  then  called  attention  to  the  drawing  of  a 
bridge  with  truss  rods  to  every  upright  post.  The  span  was 


^ 45939 


4 


AMERICAN  IRON  BRIDGES. 


125  feet,  and  tlie  depth  was  23  feet  from  the  centre  of  the 
top  chord  to  the  centre  of  the  bottom  chord ; yet  all  the  iron 
posts,  the  truss  rods,  the  diagonal  tension  rods,  and  the  lean- 
ing end  columns,  were  not  together  equal  to  a quarter  of  an 
inch  in  thickness  of  iron,  if  spread  out  as  a plate,  oyer  the 
whole  side,  and  half  of  that  thickness  was  cast-iron  ; still  no 
portion  of  the  iron  was  subjected  to  a greater  working  strain 
than  4 tons  to  the  square  inch.  Under  ordinary  circum- 
stances a working  strain  of  3 tons  to  the  square  inch  was  the 
utmost  that  existed.  He  contrasted  with  that  design  some 
plate  girder  bridges,  erected  three  years  ago,  on  the  Boston 
and  Worcester  Bailway.  They  were  87  feet  span  and  7 feet 
6 inches  deep.  The  plates  were  6 feet  3 inches  wide  and  7 
feet  6 inches  high.  At  every  vertical  joint  there  was  a pair 
of  butt  straps,  8 inches  wide,  double  riveted  on  each  side  of 
the  web,  and  over  these  a pair  of  angle  irons,  3 inches  by  6 
inches.  Midway  between  the  vertical  joints,  there  were  two 
angle  irons,  6 inches  by  3 inches,  the  longer  sides  being 
turned  over  at  right  angles,  to  form  a knee  by  which  they 
were  riveted  to  the  top  and  bottom  chords.  Mr  Philbrick, 
the  engineer,  adopted  that  form  of  stiffening  the  webs  of 
girder  bridges,  because,  as  he  said,  he  had  found  that  in 
England  and  in  France,  the  T irons  used  for  stiffening  had 
begun  to  split  in  some  cases.  He  had  only  called  attention 
to  the  butt  straps  and  angle  irons  used*  by  Mr.  Philbrick,  be- 
cause these,  spread  out  as  plates  over  the  side  of  the  web, 
were  alone  equal  to  one-quarter  of  an  inch  in  thickness, 
while  in  the  Murphy- Whipple  Bridge  the  whole  of  the  mem- 
bers forming  the  web  amounted  to  less  than  a quarter  of  an 
inch. 

He  might  add,  that  none  of  the  bridges  shown  in  the 
drawings  had  broken  down,  and  he  had  never  heard  that 
they  had  exhibited  any  signs  of  weakness.  They  were  con- 
sidered in  America,  and  he  believed  them  to  be,  very  strong, 
and  good  bridges.  If  they  had  any  peculiar  merit,  it  might 
be  in  the  fact  that  they  were  not  overloaded  with  ballast. 
The  trusses  were  deeper,  and,  therefore,  there  was  not  so 
much  iron  put  into  the  sides  as  might  be  expected  to  keep 
the  strains  down. 

In  Bollman’s  Bridge  the  test  load  was  1^  ton  per  lineal 
foot,  for  a single  line.  In  America  the  assumed  load  was 
arrived  at,  by  supposing  that  there  was  on  the  bridge  a train 
of  loaded  goods  wagons,  with  two  engines,  and  a snow- 
plough weighing  about  15  tons ; and,  in  addition,  when  the 
bridge  had  a floor,  that  this  was  covered  with  snow,  equal  to 
a weight  of  30  lbs.  per  square  foot  of  floor  ; and  that  a side 


AMERICAN  IRON  BRIDGES. 


5 


wind,  equal  to  30  lbs.  per  square  foot,  was  blowing.  It  was 
further  assumed  that,  as  every  wheel  had  a break,  all  the 
breaks  were  put  on  at  once,  which  would  throw  the  strain 
upon  the  top  chords  of  the  bridge.  That  was  the  assumed 
load  of  the  bridge  of  200  feet  span,  and  it  was  adopted  by 
Mr.  Murphy,  Mr.  Whipple,  and  Mr.  Bollman. 

In  answer  to  inquiries  from  several  members,  Mr.  Colburn 
said  that  the  highest  breaking  strain  of  the  cast-iron  alluded 
to  in  the  Paper  was  20J  tons  per  square  inch  ; ‘but  it  was 
certainly  never  subjected  to  that  strain  in  a bridge.  It  was 
boiled  cast-iron,  that  was  melted  and  kept  in  fusion  for  sev- 
eral hours,  and  partially  decarbonized.  Previous  to  the 
present  war  in  America,  every  cast  gun  was  made  of  iron 
melted  two,  three,  or  four  times. 

Mr.  F.  J.  Bramwell  regretted  to  hear  that  the  arrange- 
ments for  the  reading  of  other  communications  were  such, 
that  it  was  desirable  to  limit  the  present  discussion  ; as  he 
feared  there  would  be  great  difficulty  in  investigating  the 
principles  of  construction  of  the  Trusses  described  in  the 
Paper,  unless  some  considerable  time  were  devoted  to  that 
purpose. 

With  regard  to  the  principles  involved  in  the  construction 
of  the  Trusses  of  American  Iron  Bridges,  he  referred  to 
three  diagrams  he  had  caused  to  be  prepared,  showing,  re- 
spectively, the  Fink  truss  (Fig.  1),  an  ordinary  diagonal  truss 
which  was  so  well  known  in  this  country  (Fig.  2),  and  the 
Bollman,  or  Harper’s  Ferry  Bridge,  truss  (Fig.  3).  In  these 
diagrams,  one-half  of  each  figure  was  drawn  merely  in  lines, 
running  along  the  centres  of  the  various  members  of  the 
truss  ; while  in  the  other  half  of  each  figure  the  members 
were  indicated  either  by  dotted  fines  simply,  or  by  dotted 
fines  and  shading  on  each  side,  so  as  to  show,  by  the  width 
of  the  shading  or  of  the  dotted  lines,  the  relative  amounts  of 
iron  required  jn  each  member  ; and  by  the  different  kinds  of 
dotted  lines  and  the  different  kinds  of  shading,  to  distinguish 
between  the  parts  in  compression  and  those  in  tension  ; dis- 
carding, for  the  sake  of  simplicity,  all  consideration  of  rivet 
holes,  joints,  and  such  matters,  and  assuming  the  whole  of 
the  iron  to  be  brought  to  an  uniform  thickness,  and  to  be 
equally  valuable  for  compression  and  for  extension.  Thus, 
if  the  shading  along  one  member  of  the  truss  were  twice  as 
wide  as  along  another,  then  it  indicated  that  there  must  be 
double  the  sectional  area  of  metal  in  the  first  of  these  two 
members,  that  there  was  required  to  be  in  the  second  of  such 
members.  In  this  way  the  diagrams  would  at  once  instruct 
the  eye,  as  to  the  consumption  of  metal  in  each  part  of  each 


6 


AMERICAN  IRON  BRIDGES. 


truss,  while  the  difference  of  the  shading  would  show  which 
part  was  in  compression  and  which  in  tension.  Further,  at 
the  end  of  each  figure  a parallelogram  had  been  drawn, 
which  indicated  the  relative  amount  of  each  kind  of  metal 
required  in  the  three  different  trusses.  Assuming  that  there 
would  be  no  special  circumstances  of  convenience,  or  incon- 
venience, attending  the  manufacture  of  any  particular  truss, 
there  could  be  no  question  that  was  the  best  truss  which 
consumed  'the  least  metal  in  its  construction.  And  if  the 
decision,  as  to  the  merits  of  the  Fink  truss  (Fig.  1),  and  of 
the  Bolhnan  truss  (Fig.  3),  were  to  be  arrived  at  by  the  con- 
sideration of  this  test,  Mr.  Bramwell  believed  he  was  cor- 
rect in  saying,  that  they  were  both  of  them  inferior  to  the 
ordinary  diagonal  truss  (Fig.  2),  and  in  the  following  ratio  : — 

Diagonal  truss  required  262  parts  of  metal. 

Fink  “ “ 896 

Bollruan  “ “ 370 

The  distribution  of  the  metal  would  be  as  follows  : — 


Diagonal  Truss  : Compression 136 

Tension 126 

262 

Fink  Truss  : Compression 200 

Tension 196 

396 

Bollman  Truss  : Compression 185 

Tension 185 

370 


These  results  were  arrived  at  by  a calculation  which  he 
believed  to  have  been  based  on  correct  principles,  and  to 
have  been  accurately  worked  out.  He  would,  however,  en- 
deavor to  explain  the  causes  of  the  extra  consumption  of 
iron  in  the  Fink  and  the  Bollman  trusses. 

First,  as  to  the  Fink  truss  : — The  diagram  was  made  on 
the  assumption,  that  the  depth  of  this  truss,  and  of  course 
of  the  other  two  trusses  also,  was  one-eighth  of  the  span, 
and  that  the  truss  was  loaded  with  fifteen  units  of  load,  one 
over  each  of  the  vertical  struts.  The  stmts  on  the  left-hand 
side  were  lettered  from  A to  H,  the  latter  letter  being  over 
the  central  strut.  The  effect  of  these  units  of  load  would  be, 
that  unit  A would,  by  its  vertical  A /,  be  carried  one-half  by 
the  tie-rod  / a,  and  one-half  by  / B ; and  that  the  unit  B 
would  be  carried  on  its  vertical  B b}  which  would  also  have 


AMERICAN  IRON  BRIDGES. 


7 


to  carry  the  half  of  unit  A,  brought  on  by  the  tie-rod  f B, 
and  also  one-half  of  the  unit  C,  brought  on,  in  a similar 
manner,  by  the  tie-rod  g B.  In  this  way  the  vertical  B b 
would  get  a load  of  two  units,  one-half  of  which,  or  one  unit, 
would  have  to  be  upheld  through  the  tie-rod  b a,  but  as  the 
upper  part,/  a,  of  this  tie  was  already  strained  by  the  weight 
due  to  one-half  of  the  unit  A,  and  as  the  angle  of  the  tie 
was  45°,  the  tie  b a would  bring  a compression  of  exactly  a 
unit  and  one-half  on  the  top  member  of  the  girder.  It  was 
clear  that  a downward  load  of  one  unit  and  a half  must  also 
be  brought,  by  the  tie  b g D,  on  to  the  point  D,  and  that  a 
similar  load  of  a unit  and  a half  must  be  brought  on  to  the 
same  point  by  the  tie  d h D,  making  together  a load  of  three 
units.  This,  added  to  the  unit  D itself,  would  cause  a load 
of  four  units  to  have  to  be  carried  by  the  vertical  D c.  Of 
these  four  units  two  would  be  carried  by  the  tie  c a,  and  the 
other  two  by  the  tie  c H.  The  tie  c a would  bring  the  com- 
pression on  to  the  top  member  of  the  girder  due  to  these 
two  units.  If  the  tie  c a had  been  at  the  same  angle  as  the 
tie  bf  a,  this  compression  would  only  have  been  two  units  ; 
but  as  the  tie  c a was  at  an  angle  only  half  as  favorable  as 
the  tie  bf  a,  the  compression  brought  on  to  the  top  member 
of  the  truss  by  the  tie  c a would  be  double  that  of  the  verti- 
cal load  carried  by  it,  or  a compression  of  four.  The  last 
source  from  which  compression  would  come  upon  the  top 
member  of  the  truss  would  be  from  the  strain  of  the  tie  e a. 
This  tie,  it  would  be  seen,  had  to  bear  one-lialf  the  duty  of 
upholding  the  central  strut  H e.  This  strut  H e would  have 
to  support,  first,  two  units  of  load  brought  on  it  by  the  tie 
c H ; secondly,  one  unit  of  load  brought  on  by  the  tie  d i H, 
owing  to  that  tie  upholding  one-half  of  the  load  of  two  on 
F d,  and,  thirdly,  one-half  of  a unit  of  load  brought  on  by 
the  upper  part  i H of  the  same  tie  d i H,  which  upper  part 
would  have  to  support  the  half  of  the  unit  of  load  on  G i. 
These  three  sources  of  strain  would  amount  together  to  three 
and  a half  units.  There  would,  of  course,  be  an  equal  load 
brought  on  by  the  corresponding  ties  to  the  right  hand  of  the 
strut  H e,  which  strut  would  thus  be  loaded  with  seven  units, 
„ to  which  must  be  added  the  unit  on  H itself,  making  eight 
as  the  total  load  on  the  strut  H e.  Of  this  eight,  one-half, 
or  four,  would  be  upheld  by  the  tie  e a,  and  by  this  tie  would 
exert  a compressive  force  on  the  top  member  of  the  girder. 
It  would  be  seen,  that  the  angle  at  which  this  tie  e a lay  was 
twice  as  unfavorable  as  that  of  the  tie  last  considered,  and 
four  times  as  unfavorable  as  that  of  the  tie  bf  a,  the  angle 
of  which  was  45°.  Owing  to  this  unfavorable  angle  of  the 


8 


AMERICAN  IRON  BRIDGES. 


tie  e a,  the  load  supported  by  it  would  exert  a compression 
of  four  times  its  own  amount,  or  sixteen.  The  sum  of  the 
compressions,  therefore,  on  the  top  member  of  the  girder 
would  be  : — 

For  the  tie/  a £ unit  x by  the  effect  of  the  angle  1 = \ 

“ “ ha  1 “ “ “ “ 1=  1 

“ “ c a 2 “ “ “ “ 2—  4 

“ « ea4  t(  “ “ “ 4 = 16 

/ 


It  would  be  seen,  that  this  compression  extended  from  end 
to  end  of  the  top  member  of  the  truss,  so  that  the  top  mem- 
ber would  require  to  have  as  great  a sectional  area  at  the 
extremities  as  in  the  middle.  Assuming  that  one  part  of  iron 
was  required  to  resist  one  unit  of  load,  then  the  sectional 
area  would  be  21J,  and  the  length  of  the  truss  being  taken 
as  8,  the  amount  of  iron  consumed  in  the  compression  bar 
would  be  equal  to  21J  x 8,  or  172  parts  of  iron. 

Before  considering  the  amount  of  iron  in  the  other  mem- 
bers of  the  Fink  truss,  Mr.  Bramwell  would  compare  the 
compression  bar,  or  top  member,  of  this  truss,  with  that  of 
an  ordinary  diagonal  truss  (Fig  2).  In  the  latter  case  it  was 
well  understood  that,  as  fifteen  units  of  load,  distributed  as 
shown,  were  equal  to  a .central  load  of  eight,  and  as  the 
girder  was  eight  times  as  long  as  it  was  deep,  the  compres- 
sion in  the  centre  would,  of  course,  be  sixteen  units  of  load. 
This,  then,  would  represent  the  maximum  sectional  area, 
which  would  exist  only  at  the  very  middle  of  the  compres- 
sion member,  as  it  would  become  less  and  less  towards  the 
ends  of  the  girder ; the  diminution  being  in  such  a ratio,  if 
the  number  of  points  of  attachment  of  the  ties  and  struts 
were  infinite,  as  would  give  a parabolic  curve,  the  area  of 
which  would,  <5f  course,  be  two-thirds  of  the  parallelogram 
containing  it.  Therefore  16  x § = 10§  would  represent  the 
average  depth  of  the  compression  member,  which  multiplied 
by  the  length  8,  would  give  85^  as  the  amount  of  iron  in  that 
member,  or  rather  less  than  one-half  of  the  iron  in  the  sijni- 
lar  member  of  the  Fink  truss.  In  the  diagonal  truss,  with 
the  number  of  ties  and  struts  as  drawn,  this  theoretic  amount 
was  diminished  only  to  the  extent  of  1^ ; that  was  to  say, 
the  whole  amount  of  iron  was  84,  arrived  at  thus  : — 


AMERICAN  IRON  BRIDGES. 


9 


Pressure 


Between  the  points  a A the  compression  would  be  3£  units,  which 
plied  by  the  effect  of  the  angle  , 


“ * AB  “ 10£ 

“ B C “ 16i 

“ CD  “ 21i 

“ DE  “ 25£ 

“ E F “ 283 

“ PG  “ 30£ 

“ GH  “ 31  k 


, multi- 

t 


II 

54 

84 

104 

124 

144 

154 

154 


Giving  for  the  sum  of  the  compressions 84 


This  divided  bj  the  number  of  spaces  8,  made  10|  as  the 
average  depth. 

It  was,  he  thought,  not  improbable  that  a cursory  view  of 
the  drawing  of  the  Fink  truss  might  lead  to  the  idea  that  it 
was  one  that  must  be  economical ; inasmuch  as  it  appeared 
to  have  a compression  member  only,  and  to  be  without  any 
corresponding  formal  tension  member,  the  diagonal  ties  be- 
ing made  to  serve  the  purpose  of  a regular  tension  member. 
But  even  if  this  were  so,  and  if  the  ties  were  made  at  no 
greater  consumption  of  metal  than  the  ties  and  struts  of  a 
diagonal  girder,  which  he  would  prove  was  not  the  case,  even 
then  the  Fink  truss  would  have  no  advantage  ; inasmuch  as 
its  one  compression  member  contained  slightly  more  iron 
than  the  compression  member  and  tension  member  together 
of  the  diagonal  truss.  He  had  stated  that  the  compression 
member  of  this  latter  girder  contained  84  parts.  The  ten- 
sion member  contained  rather  more,  thus  : — 

Tension. 

Between  the  points  b c the  tension  would  be  4 units,  which,  multiplied 

by  the  effect  of  the  angle  = 2 


a 

c d 

66 

11 

66 

5 h 

n 

d e 

66 

17 

66 

H 

a 

ef 

66 

22 

11 

cc 

f g 

66 

26 

6 6 

13 

a 

gh 

66 

29 

66 

14  h 

a 

h i 

66 

31 

15  h 

66 

i k 

66 

32 

16* 

Giving  for  the  sum  of  the  tensions 86 


This,  divided  by  their  number,  8,  made  10|  as  the  average 
depth.  The  amount  of  metal,  86,  in  that  member,  added  to 
that  in  the  compression  member,  gave  170  parts  for  the 
metal  in  those  two  members,  as  compared  with  172  parts  in 
the  compression  member  alone  of  the  Fink  truss. 

He  would  now  revert  to  the  consideration  of  the  Fink 
truss  : — 


10 


AMERICAN  IRON  BRIDGES. 


It  would  be  remembered  that  the  main  tie  bar  e a had  to 
transmit  four  units  of  load  ; but  these  became  multiplied  in 
their  power  of  strain  on  the  tie,  by  the  number  of  times  the 
length  of  that  tie  exceeded  the  depth  of  the  girder.  This 
depth  being  1,  and  the  half  length  of  the  girder  being  4,  the 
length  of  the  tie  would  of  course  be  equal  to  Vl2  + 42='v/17. 
The  strain,  therefore,  would  be  the  load  y Vl7.  But  as  the 
length  of  the  tie  was  also  Vl7,  it  followed  that  the  amount 
of  metal  must  be  Vl7  x -\/l7  x the  load  of  4 ; in  other 
words,  the  amount  of  metal  increased  as  the  square  Parts  of 
of  the  length  of  any  sloping  tie.  In  this  case  the  Metai. 


amount  was  2 ties  each  17x4  = 136 

Similarly,  it  could  be  shown,  that  the  ties  ca , cH, 
and  their  Counterparts  on  the  other  side  of  the 
truss,  would  require  V5  x V5  x 2 load  x 4 of 

them  = . 40 

That  ba,  bT>,  d D,  (7H,  and  their  counterparts  on  the 
other  side  of  the  truss,  would  require  V2  x V2  x 1 

load  x 8 of  them  = ..... 16 

and  that  the  upper  parts  of  these  ties,  fa,  gD,  hD, 

^'H,  and  their  counterparts  on  the  other  side  of  the 


truss,  with  the  corresponding  short  ties,  / B,  gB, 
hF,  iF,  and  their  counterparts  on  the  other  side 
of  the  truss,  would  require  Vj  x Vj  x J load  x 16 
of  them  = 4 


Making  the  total  of  iron  in  tension,  as  already  stated  196 


Further,  the  compression  member,  it  had  been  PMetai!f 


proved,  would  contain 172 

The  strut  He  would  contain  1x8  units  = 8 

The  strut  D c,  and  the  corresponding  one  on  the 
other  side  of  the  truss,  would  contain  1x4  units 

x 2 of  them  = 8 

The  struts  B b,  F d,  and  their  corresponding  ones  on 
the  other  side  of  the  truss,  would  contain  1x2 

units  x 4 of  them  = 8 

The  struts  A/,  Cg,  E li,  Gf,  and  their  corresponding 
ones  on  the  other  side  of  the  truss,  would  contain 
^ long  x 1 unit  x 8 of  them  == 4 


Making  the  total  of  iron  in  compression,  as  already 

stated 200 


AMERICAN  IRON  BRIDGES. 


11 


In  order  to  complete  the  comparison  between  the  Fink 
truss  and  the  diagonal  truss,  it  would  be  necessary  to  ascer- 
tain the  amount  of  metal  in  the  struts  and  ties  of  this  latter 
truss.  The  lengths  of  the  various  struts  and  ties  would  be 
uniformly  Vl*,  the  multiple  of  strain  would  therefore  be  Vl  j, 
and  the  amount  of  iron  in  each  of  them  1J  x the  units  of 
load : — 

On  the  strut  A b there  would  be  4 units. 


tt 

B c 

(t 

3J 

a 

it 

C d 

a 

3 

a 

it 

D e 

(( 

2J 

a 

tt 

E/ 

tt 

2 

a 

tt 

E 9 

tt 

li 

a 

tt 

Gh  • 

tt 

l 

a 

tt 

Hi 

a 

1 

2 

a 

Making  a total  of 18 


This  sum  multiplied  by  the  1 j,  would  give 22 

To  which  must  be  added  the  end  upright  ah = 3 

Making 26 


Or,  for  the  two  sides  of  the  truss 52 

Adding  the  amount  of  metal  in  the  compression  mem- 
ber before  given 84 


The  total  of  iron  in  compression  would  then  be, 
as  already  stated . 136 


Next,  as  regarded  the  parts  in  tension  : — 


On  the  tie  a c there  would  be  34  units. 


vyii  me  tiu 

a 

Uj  V lilfcJI 

Ad 

tJ  WUUJLU 

ft 

3 

uiiil». 

tt 

a 

B c 

• tt 

2i 

tt 

a 

c/ 

it 

2 

tt 

a 

Vg 

tt 

li 

it 

a 

E h 

tt 

i 

tt 

a 

F i 

tt 

1 

2 

tt 

it 

Gh 

tt 

0 

tt 

Making  a total  of 

This,  multiplied  by  1|,  would 

give. . . , 

14 

12 


AMERICAN  IRON  BRIDGES. 


Making  for  the  two  sides  of  the  truss 35 

Adding  the  amount  of  metal  in  the  tension  member  be- 
fore given 86 


The  total  of  metal  in  tension  was  thus  found  to  be ... . 121 

4 

But  to  this  should  be  added  the  ties  G 1c,  which,  although 
not  called  into  action,  when  an  uniform  load  was  on  the 
girder,  would  have  to  bear  a strain  if  the  load  were  a 
passing  one.  They  might  then  be  required  to  support  a 
single  unit  of  load.  Under  similar  circumstances  two 
ties  on  each  side  might  require  to  be  strengthened,  so  as 
to  bear  a further  half  unit  each ; then  4 ties  1 unit  x 

thelj = 5 

• 

Giving  as  the  total  of  iron  in  tension,  as  already  stated . 126 


He  would  not  occupy  any  further  time,  by  entering  into 
the  details  of  the  calculation  of  the  quantity  of  metal  in  the 
Bollman  Truss  ; but  would  simply  refer  to  the  figures  already 
given,  by  which  it  would  be  seen,  that  while  it  was  more 
economical  than  the  Fink  Truss,  in  the  proportion  of  37  to 
39.6,  it  was  less  economical  than  the  ordinary  diagonal  truss, 
in  the  proportion  of  37  to  26.2. 

The  cause  of  the  waste  of  metal  in  the  Fink  and  in  the 
Bollman  trusses  appeared  to  him,  on  a little  consideration, 
to  be  sufficiently  obvious.  The  strains  which  related  to  the 
centre  part  only  of  a truss,  and  which  might  be  got 
rid  of  in  a comparatively  short  distance  near  that  centre, 
were,  in  the  Fink  and  in  the  Bollman  trusses,  carried  to  the 
very  ends  of  the  compression  member,  so  that  the  whole 
length  of  that  member  sustained  a pressure  that  need  only 
be  borne  near  the  centre.  Further,  to  carry  out  this  bad 
arrangement,  the  tie-rods  were,  of  necessity,  placed  at  most 
unfavorable  angles,  by  which  not  only  was  an  unnecessary 
amount  of  metal  consumed,  but  the  already  useless  strain  on 
the  compression  member  was  aggravated. 

It  was  unnecessary  to  investigate  the  construction  of  the 
Murphy- Whipple  truss,  as  the  drawing  showed  that  truss  to 
be  free  from  the  radical  error,  of  carrying  the  whole  of  the 
compression  to  the  very  ends  of  the  top  member.  It  was, 
however,  clear,  that  the  use  of  struts  in  a vertical  position 
was  not  so  economic  as  the  use  of  struts  in  a diagonal  posi- 
tion, inasmuch  as  the  struts  when  placed  vertically  did  not 
assist  in  the  progression  (if  such  a term  might  be  allowed j 


AMERICAN  IRON  BRIDGES. 


13 


from  end  to  end  of  the  truss.  The  ties,  also,  were  not  dis- 
posed at  the  most  economic  angle  ; but  with  those  excep- 
tions, he  had  little  doubt  the  Murphy- Whipple  truss  would, 
on  investigation,  prove  to  be  one  of  good  construction,  and 
equal  to  such  as  were  in  ordinary  use  in  England. 

He  regretted  having  to  pronounce  so  unfavorable  an  opin- 
ion on  these  trusses  ; but  he  was  glad  to  be  enabled  to  do 
so,  without  the  fear  of  his  criticism  being  displeasing  to  the 
Author,  who,  it  was  understood,  had  merely  submitted  these 
particular  trusses,  as  matters  of  interest  in  the  history  of, 
railway  works  in  America,  without  expressing  approval  of 
their  construction,  and  for  the  very  purpose  of  having  their 
merits,  or  demerits,  fully  discussed. 

Mr.  Phipps  said,  the  Paper  was  for  the  most  part  limited 
to  several  varieties  of  trussed  girders.  On  these  he  would 
not  at  present  offer  any  observations,  but  would  confine  his 
remarks  to  one  or  two  iron  arched  constructions,  also  referred 
to  in  the  Paper,  as  he  had  paid  particular  attention  to  the 
subject  of  cast-iron  arches.  It  had  been  truly  remarked, 
that  whenever  these  arched  constructions  were  loaded  irreg- 
ularly, it  became  difficult  to  calculate  the  strain  to  which  the 
material  then  became  subject.  For  instance,  iron  arches, 
particularly  on  railways,  might  be  loaded  upon  one-half  of 
the  span  only.  In  such  a case  the  curve  of  equilibrium 
would  often  shift  so  much  from  the  middle  of  the  arched  rib, 
as  almost  to  touch  the  extrados  and  the  intrados  of  the  arch 
on  opposite  sides  of  the  centre.  Having  obtained,  however, 
the  position  of  the  curve  of  equilibrium,  it  then  became  a 
question,  how  to  estimate  the  effect  of  the  pressure  in  its 
detrimental  action  on  the  outer ‘fibres  of  the  rib.  This  was 
a point  upon  which,  he  agreed,  that  no  practical,  or  reliable, 
information  was  to  be  obtained  from  books,  and,  in  conse- 
quence, he  had  invented  a method  which  he  had  found  both 
simple  and  accurate  in  its  application.  To  illustrate  the  de- 
gree to  which  the  shifting  of  the  line  of  pressure  might  affect 
the  strain  upon  an  iron  arch,  when  placed  under  compression, 
he  stated  that  in  a prismatic  bar  of  iron,  in  shape  like  a 
3-inch  plank,  the  pressure  on  the  outer  fibres  on  one  side 
would  be  doubled,  and  on  the  other  side  be  reduced  to  noth- 
ing, by  the  removal  of  the  line  of  pressure  only  the  one-sixth 
part  of  the  whole  width  away  from  the  centre  of  gravity. 
Now  in  this,  as  in  every  other  case,  whenever  a piece  of 
metal,  or  other  elastic  material,  was  compressed  by  a force 
on  any  other  line  than  that  of  its  centre  of  gravity,  the  action 
upon  the  outer  fibres  might  be  obtained,  by  conceiving  the 
material,  first  of  all  to  be  compressed  squarely  throughout, 


14 


AMERICAN  IRON  BRIDGES. 


as  if  the  strain  were  applied  over  the  line  of  centre  of  grav- 
ity, and  then,  finding  the  strain  due  toHhe  angular  motion, 
by  the  same  process  that  would  be  used  for  obtaining  the 
strain  on  that  portion  of  a cantilever  contiguous  to  where  it 
was  attached  to  a wall ; using  the  total  compressive  force 
applied  as  the  weight,  and  the  distance  of  that  pressure  from 
the  line  of  centre  of  gravity  as  the  leverage.  Then  the 
strain  on  the  outer  fibre  thus  obtained,  added  to  the  former 
square-on  pressure,  would  give  the  whole  detrimental  action 
upon  the  outer  fibre  of  the  material.  Thus,  as  he  had  pre- 
viously explained  in  reference  to  the  Charing  Cross  Bridge, 
when  a pressure  of  475  tons  was  applied,  square-on,  to  a 
piece  of  iron  of  161.25  square  inches  sectional  area,  all  the 
fibres  would  be  strained  uniformly  up  to  2.94  tons  on  the 
square  inch  ; but  by  removing  the  line  of  pressure  3.6  inches 
away  from  the  centre  of  gravity,  the  pressure  on  the  outer 
fibres  would  be  increased  up  to  6.86  tons  on  the  square  inch. 
Mr.  Phipps  added  that,  in  cases  where  the  arched  rib  proper 
was  well  connected  with  the  roadway  bearer  above,  by  an 
efficient  system  of  diagonal  bracing,  the  centre  of  gravity  of 
the  whole  section  of  the  arched  rib  and  roadway  bearer  to- 
gether must  be  taken,  as  that  around  which  the  previously- 
named  angular  motion  must  be ’computed. 

Mr.  F.  W.  Sheilds  could  not  agree,  that  the  object  of  the 
bracing  in  a girder  bridge  was  merely  to  keep  the  top  and 
bottom  members  asunder,  and  to  enable  them  to  do  their 
work.  In  his  opinion,  the  chief  object  of  bracing  in  all 
parallel  girders,  was  to  carry  the  vertical  weight  of  the 
bridge,  with  the  extraneous  load  upon  it,  to  the  points  of 
support  at  the  piers  ; and  that  duty  was  just  as  important 
as,  and  quite  distinct  from,  the  work  to  be  performed  by  the 
top  and  bottom  flanges.  The  bracing,  ought  to  be  specially 
designed  for  that  duty,  and  in  that  view  he  thought  the 
American  bridges  brought  under  their  notice,  even  those  of 
the  simplest  form,  showed  a want  of  scientific  knowledge  in 
their  construction.  Thus,  if  a girder  bridg#  be  supposed  to 
consist  of  twelve  bays,  and  the  driving-wheels  of  a locomo- 
tive loaded  with  twelve  tons  rested  over  the  third  bay,  that 
load  would  be  transmitted  to  each  pier,  by  the  bracing,  in 
the  inverse  proportion  of  the  distance  of  the  loaded  point 
from  the  piers,  being  nine  tons  to  the  nearer,  and  three  tons 
to  the  further  pier.  One  of  the  diagrams  exhibited  was  just 
in  such  a case,  and  he  observed  that  at  the'  third  bay,  there 
was  no  diagonal  tie  from  the  lower  flange  which  could  carry 
the  load  to  the  further  pier.  Consequently  the  girder  would 
have  to  depend  merely  upon  the  rigidity  of  the  top  flange, 


AMERICAN  IRON  BRIDGES. 


15 


considered  as  an  independent  beam  of  perhaps  a few  inches 
deep,  for  the  transmission  of  that  part  of  the  load  which 
reached  the  further  pier,  and  which  should  be  sustained  by 
the  whole  girder  of  several  feet  deep.  Diagonals  of  great 
inclination  were  not  advisable,  independent  of  all  increase  of 
strain  arising  from  their  inclination,  as  they  were  free  to  de- 
flect on  a curve  struck  from  one  of  their  ends  as  a centre  and 
the  other  end  |ts  a radius,  and  that  curve  would  coincide  in 
practice,  for  a considerable  distance,  with  a vertical  line,  so 
that  there  was  little  or  no  resistance  to  deflection.  There 
were  some  bridges  in  the  Regent’s  Park  upon  that  principle, 
which  fully  bore  out  his  foregone  conclusions,  for  he  found 
that  by  jumping  upon  those  bridges,  the  whole  could  be  set 
in  motion  by  his  own  weight  alone. 

Mr.  Zerah  Colburn  remarked,  with  reference  to  the  com- 
parison which  had  been  made  between  the  various  trusses, 
that  it  was  well  understood  in  the  States,  as  applying  to  the 
Fink  and  B oilman  Bridges.  The  Murphy- Whipple  Bridge 
was  believed  to  be  the  best  class  known  in  the  States  ; and 
if  there  was  not  much  difference  between  that  and  many 
bridges  in  this  country,  it  was,  no  doubt,  owing  to  the  fact, 
that  when  the  principles  were  fixed,  there  could  not  be 
much  room  for  difference  between  one  good  bridge  and 
another. 

Mr.  Hawkshaw,  President,  expressed  his  regret  that,  owing 
to  the  late  period  of  the  Session,  and  the  necessity  for  read- 
ing two  other  Papers  before  its  close,  the  discussion  upon  a 
subject  of  so  much  general  interest  as  American  Iron 
Bridges  should  have  been  necessarily  restricted.  The  Insti- 
tution was  under  great  obligations  to  Mr.  Colburn  for  the 
able  and  elaborate  manner  in  which  the  communication  had 
been  made  ; and  he  hoped  that  the  Author  would,  on  other 
occasions,  contribute  to,  and  take  part  in,  the  proceedings. 


A 


I 


■ 

. 

I 


£ 


X 


A 


f 


